The smallest two-digit number N = ab such that the sum and absolute difference
of its digits along with the concatenations of each in both directions are all prime.
E.g., 5 - 2 = 3, 5 + 2 = 7, 37, and 73 are prime. [Heleen]

(100) = 25.

There are exactly 52 squares less than or equal to 10002 that are the sum of twin prime pairs. [Honaker]

25 is the concatenation of 2 and 5 which are the only primes of the form 10n + 2 and 10n + 5. Note that 25 = 52. [Murthy]

Laura Nickel, a high school student when she made international news by codiscovering the 25th Mersenne prime, is now Ariel Glenn.

There are exactly 25 (a perfect square) primes beginning with the digit 1 among primes smaller than one thousand. It's more than for the other digits. [Capelle]